Right-wing youth activist and influencer Charlie Kirk, a major ally of President Donald Trump, has been shot dead in an apparent assassination that sparked fears of more political violence in an increasingly febrile United States.

Trump confirmed on social media that Kirk, 31, had died from his injuries.

“The Great, and even Legendary, Charlie Kirk, is dead,” Trump wrote on Truth Social.

Kirk was hit while speaking at an event at Utah Valley University.

Video from the scene showed him addressing a large crowd when the sound of a single shot rang out. Kirk appeared to collapse in his chair before the camera swiftly moved and sounds of panic erupted in the audience.

The university said the shooting happened around midday (local time) and initially said that a suspect was in custody, although multiple US media reports later said that was not the case.

Illustrating the huge importance of Kirk in Trump’s orbit, the 79-year-old Republican ordered flags on Government buildings to be lowered to half-staff until Sunday. American flags at the White House were lowered shortly after the President’s order.

“No one understood or had the Heart of the Youth in the United States of America better than Charlie. He was loved and admired by ALL, especially me, and now, he is no longer with us,” Trump wrote.

“Melania and my Sympathies go out to his beautiful wife Erika, and family. Charlie, we love you!”

‘He fell back’

Former Utah congressman Jason Chaffetz, who was at the rally, told Fox News the shooting had happened while Kirk was doing a question and answer session with the crowd.

“First question was about religion. He went on for about 15-20, minutes. Second question, interestingly, was about transgender shooters, mass shooters, and in the midst of that, the shot rang out,” a visibly shaken Chaffetz told the network.

“As soon as that shot went out, he fell back,” he said. “Everybody hit the deck, everybody... a lot of people started screaming, and then everybody started running.”

Sophie Anderson, 45, told the Daily Mail she was standing 30m from the stage, said she knew it was a gunshot as soon as she heard the noise.

“He was shot in the neck and just fell over and he was just a fountain of blood,” she told the outlet.

Kirk had an outsized influence in US politics, helping the rise of Trump’s support among younger voters – one of the key factors in the Republican’s return to power last year.

With natural showmanship skills, Kirk co-founded Turning Point USA in 2012 to drive conservative viewpoints among young people, turning him into the go-to spokesman on television networks and at conferences for the youthful hard-right.

He used his enormous audiences on Instagram and YouTube to build support for anti-immigration policies, outspoken Christianity, and viral takedowns of hecklers at his many campus events.

His presence on campuses was seen on the right as a welcome contrast to widespread liberal viewpoints in higher education, but have sparked often fierce opposition.

US President Donald Trump says Charlie Kirk was "loved and admired by ALL". Photo / Getty Images

News of the shooting provoked horror across the political spectrum, with senior figures on the right paying tribute.

“Dear God, protect Charlie in his darkest hour,” wrote Vice-President JD Vance on social media, alongside a picture of the two men and the president’s son, Donald Trump jnr.

Right-wing media figure Tomi Lahren called Kirk “visionary”.

“He’s actually opened the door for so many young conservatives to have a career and have, you know, a voice in this business.

The American flags near the White House on the North Lawn fly at half staff following the death of Turning Point USA founder and conservative activist Charlie Kirk. Photo / Getty Images

Figures on the left condemned the attack and urged calm.

California Governor Gavin Newsom – a frequent foil for right-wing figures like Kirk – called the attack “disgusting, vile, and reprehensible”, while former Vice-President Kamala Harris said “political violence has no place in America”.

Former Congresswoman Gabrielle Giffords – a Democrat who survived an assassination attempt herself – said she was “horrified” by the attack.

“Democratic societies will always have political disagreements, but we must never allow America to become a country that confronts those disagreements with violence,” she said.

– Agence France-Presse